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06

The Mole Concept and Stoichiometry

Set BExam practice
Name
Time 1 hour
Total 35 marks

A Data Booklet may be used. Molar volume of a gas = 22.7 dm3 mol−1 at s.t.p.

A

Multiple choice

3 marks
1

The compound Bi2Sr2Ca2Cu3O10 is a superconductor. In it, the oxidation number of bismuth is +3, of strontium and calcium +2, and of oxygen −2.

What are the possible oxidation numbers of the three copper atoms?

A+1, +1, +2
B+1, +2, +3
C0, +1, +3
D0, +2, +3
Answer [1]

Source: 2025 ACJC J2 Prelim P1 Q8

2

The table shows the absolute abundances of the isotopes in a sample of naturally occurring zinc.

isotope64Zn65Zn67Zn68Zn
absolute abundance100802010

What is the relative atomic mass of this sample of zinc?

A64.0
B64.9
C68.1
D72.2
Answer [1]

Source: NJC 2018 Promo, via EJC 2022 Mole Concept Topical Revision, Q2

3

CommonA disproportionation reaction occurs when an element is oxidised and reduced at the same time. Which equation describes a disproportionation reaction?

A3ClO− → ClO3− + 2Cl−
B2H2C2O4 → 2H2O + 2C + 2CO + 2O2
C2FeSO4 → Fe2O3 + SO2 + SO3
D2KMnO4 → MnO2 + MnO + K2O + 2O2
Answer [1]

Source: IJC 2017 Promo, via EJC 2022 Mole Concept Topical Revision, Q18

B

Structured questions

32 marks
4

FA 1 is a solution of hydrogen peroxide and FA 2 is a standard solution of potassium manganate(VII). A student found that 10 drops of FA 1, when acidified, needed 82 drops of FA 2 for complete reaction. She decided that FA 1 was too concentrated, and needed to be diluted before titration against FA 2.

(a)
Construct a balanced equation for the reaction between MnO4− ions and H2O2 under acidic conditions.
[1]
(b)
Explain, with a calculation, why FA 1 would be unsuitable for titration against FA 2 in the burette. Assume 25.0 cm3 of FA 1 is used.
[2]
(c)
The student diluted v cm3 of FA 1 to w cm3 of FA 3. 25.0 cm3 of FA 3 reacted with exactly y cm3 of FA 2 of concentration 0.0200 mol dm−3. Show how her results may be used to find the concentration of H2O2 in FA 1.
[2]
(d)
The volume strength of a hydrogen peroxide solution is the volume of O2, in dm3, produced at s.t.p. from 1 dm3 of the solution, by 2H2O2 → 2H2O + O2. A commercial solution has a volume strength of 13.4. Calculate its concentration of H2O2 in mol dm−3.
[1]

Source: 2023 RI J1 Promo Section B Q3(a), (b), (d) and (e)

5

CommonA 3.20 g impure sample of calcium ethanedioate, CaC2O4, is shaken with 100.0 cm3 of 0.10 mol dm−3 acidified MnO4−. One of the products is CO2. The remaining MnO4− is reacted with excess iodide to liberate I2. A 25.0 cm3 aliquot of the resulting solution needs 24.50 cm3 of 0.20 mol dm−3 S2O32−.

2MnO4− + 10I− + 16H+ → 2Mn2+ + 5I2 + 8H2O    2S2O32− + I2 → S4O62− + 2I−

(a)
Construct an equation for the reaction between MnO4− ions and C2O42− ions.
[1]
(b)
Calculate the percentage purity of the sample of CaC2O4.
[3]

Source: 2025 ASRJC J2 Prelim P2 Q3(e)

6

The concentration of Al3+ in a water sample can be found by titration with EDTA4−: Al3+(aq) + EDTA4−(aq) → [Al(EDTA)]−(aq). A 10.0 cm3 water sample is made up to 250 cm3 with deionised water. A 25.0 cm3 aliquot needs 35.45 cm3 of 0.0200 mol dm−3 EDTA4−.

(a)
Calculate the amount of EDTA4− that reacted with the aluminium ions.
[1]
(b)
Calculate the amount of aluminium ions in the water sample.
[1]

Source: 2024 DHS J2 Prelim P2 Q1(a)

7

CommonN-chlorosuccinimide, C4H4O2NCl, is used in water purification tablets. It dissolves in water to give chloric(I) acid, HClO. The amount in a tablet is found by adding excess acidified KI(aq): HClO is reduced to Cl− by I−, which is oxidised to I2. The I2 is titrated with sodium thiosulfate: I2 + 2S2O32− → 2I− + S4O62−.

(a)
Construct a balanced equation for the reaction between HClO and acidified KI(aq).
[1]
(b)
One tablet needed 6.0 cm3 of 0.0050 mol dm−3 sodium thiosulfate. Calculate the mass of N-chlorosuccinimide in the tablet.
[2]

Source: A Level 2011 P3 Q1(c), via EJC 2022 JC1 Mole Concept Tutorial, C8

8

CommonThe percentage of iron in an ore can be found by converting all the iron into Fe2+(aq) and carrying out a redox titration. A sample of iron ore weighing 11.15 g was converted to Fe2+(aq), mixed with dilute sulfuric acid and made up to 250 cm3. 25.0 cm3 portions were titrated with 0.100 mol dm−3 potassium dichromate(VI). The average titre was 21.45 cm3.

(a)
Write an ionic equation for the reaction of Fe2+ ions with acidified Cr2O72− ions.
[1]
(b)
Calculate the total amount of Fe2+ ions in the solution made up from the iron ore.
[2]
(c)
Hence calculate the percentage by mass of iron in the sample of iron ore.
[2]

Source: A Level 2013 P2 Q2(c), modified, via EJC 2022 JC1 Mole Concept Tutorial, C9

9

When P2S5 is heated under a vacuum together with caesium sulfide, Cs2S, and sulfur, it produces an ionic compound R, which has the following composition by mass: Cs, 58.1%; P, 6.78%; S, 35.1%.

(a)
CommonCalculate the empirical formula of compound R and hence deduce its chemical formula, given that its relative formula mass, Mr, is 914.6.
[2]
(b)
Compound R contains Cs+ cations and an anion, present in a 4 : 1 ratio. Write the formula of the anion.
[1]

Source: 2024 CJC J2 Prelim P2 Q2(e)(i) and (ii)

10

Iron(III) oxide, Fe2O3, can be converted into iron metal by reacting it with carbon monoxide. Carbon dioxide is formed as the only by-product in this reaction.

(a)
Write a balanced equation for the reaction.
[1]

0.15 kg of Fe2O3 and 60 dm3 of carbon monoxide at room temperature and pressure were used in the experiment. Calculate

(b)
Common(i) the maximum mass of iron metal that can be produced,
[2]
(ii) the number of carbon dioxide molecules formed as the by-product,
[1]
(iii) the percentage yield of the experiment, if 70 g of iron metal was actually obtained.
[1]

Source: EJC 2022 JC1 Mole Concept Tutorial, A10

11
Challenge25.0 cm3 of a solution containing ethanedioic acid, H2C2O4, and sodium ethanedioate, Na2C2O4, needed 14.75 cm3 of 0.100 mol dm−3 NaOH for neutralisation. Another 25.0 cm3 needed 32.00 cm3 of 0.0205 mol dm−3 KMnO4 for oxidation in acid at 60 °C. Calculate the concentrations of ethanedioic acid and of sodium ethanedioate in the solution.
[4]

Source: RJC 2010/11 Moles and Stoichiometry Tutorial, Q15